1. Field of the Invention
The present invention relates to circuit substrates upon which electronic components are mounted. In particular, the present invention relates to a circuit substrate having thereon an equivalent resistance component, which is connected in series to an electronic component such as a capacitor, an inductor, or other suitable component.
2. Description of the Related Art
Previously, power supply circuits for supplying an operating voltage have been provided in computers and other electrical apparatuses. These power supply circuits have been implemented by circuit substrates in which a plurality of bypass capacitors (decoupling capacitors) for improving the operating stability and for speeding up the operating response are connected in parallel and mounted on the substrates. Low-capacitance laminated ceramic capacitors or large-capacitance electrolytic capacitors made from aluminum, tantalum, or other material are used as the bypass capacitors, according to the power supply capacity, the switching frequency, the circuit parameters smoothing coils used therewith, etc.
In recent years, in laminated ceramic capacitors, as a result of developments in thin-film forming technology and lamination technology of dielectrics and internal conductors, the capacitance of laminated ceramic capacitors and the capacitance of electrolytic capacitors have become approximately the same, and experiments have been carried out to replace electrolytic capacitors with laminated ceramic capacitors. However, when replacing electrolytic capacitors with laminated ceramic capacitors, since the impedance in the frequency range used with laminated ceramic capacitors is too small, disturbances occur in the output power waveform of the power supply circuits. This is due to step-shaped changes in the input voltage of, for example, a three-terminal regulator included in the power supply circuit, since the equivalent series resistance (ESR), described later, is too small.
Moreover, when using electrolytic capacitors, the impedance in the frequency range used is larger than that of laminated ceramic capacitors. However, since this impedance is larger, heat generation easily occurs, and furthermore, the smoothness of the power supply line tends to deteriorate.
Here, the relationship between the frequency used and the impedance in the power supply circuit will be described. The impedance at the power supply side as seen from the load side, that is to say, the combined impedance of the parallel circuit composed of the plurality of capacitors, becomes large at a specific frequency (parallel resonance frequency) due to a parallel resonance effect. The point at which this combined impedance becomes large is known as the anti-resonant point. The combined impedance at this anti-resonant point becomes larger as the equivalent series resistance (ESR) of the capacitor becomes smaller.
As described above, because the capacitance of laminated ceramic capacitors has increased in recent years, coupled with the fact that the equivalent series inductance (ESL) is smaller, laminated ceramic capacitors are being used to replace electrolytic capacitors such as tantalum capacitors. However, since the equivalent series resistance (ESR) in large-capacitance laminated ceramic capacitors is small, the impedance at the anti-resonant point is increased.
FIG. 1 is an equivalent circuit diagram including capacitors of, for example, a smoothing circuit mounted on a circuit substrate of the related art. On a circuit substrate B1 in FIG. 1, a first capacitor 1 and a second capacitor 2 are connected in parallel. The capacitor 1 is defined by an electrostatic capacitance C1, an equivalent series resistance R1, and an equivalent series inductance L1. The capacitor 2 is defined by an electrostatic capacitance C2, an equivalent series resistance R2, and an equivalent series inductance L2. Furthermore, the circuit pattern on the circuit substrate has an equivalent inductance L, resistance R, and so on. Reference character S indicates a power supply voltage generating circuit which generates a power supply voltage. The power supply voltage generating circuit supplies a voltage to the smoothing circuit on the circuit substrate B1. Reference character T indicates a load to which the voltage from the power supply voltage generating circuit S is applied via the smoothing circuit. In FIG. 1, the power supply voltage generating circuit S is provided separately from the circuit substrate B1, however, the power supply voltage generating circuit S may be mounted on the circuit substrate B1.
FIG. 2 is a graph showing the relationship between the frequency and the impedance in the smoothing circuit (parallel capacitor circuit) shown in FIG. 1. As shown by line a in FIG. 2, a peak impedance Z1 is generated in this smoothing circuit at the parallel resonance frequency (anti-resonant point) F1.
Accordingly, setting the equivalent series resistance (ESR) of the capacitor to an appropriate value can be considered as one method to suppress the generation of such a peak impedance. In this case, since the necessary equivalent series resistance varies depending on the combined capacitors, it is necessary to arrange many equivalent series resistances so that they are suitable for all combinations. However, it is difficult to arrange this, if not almost impossible.
On the other hand, it is possible to set (regulate) the combined impedance at the anti-resonant point even when a chip resistor is mounted on the circuit substrate in series with the capacitor.
FIG. 3 is an equivalent circuit diagram including, for example, smoothing circuit capacitors and chip resistors, which are mounted on the above-described circuit substrate. A series circuit defined by the capacitor 1 and a chip resistor 3 and a series circuit defined by the capacitor 2 and a chip resistor 4 are connected in parallel on a circuit substrate B2 in FIG. 3. The capacitor 1 is defined by an electrostatic capacitance C1, an equivalent series resistance R1, and an equivalent series inductance L1, and the capacitor 2 is defined by an electrostatic capacitance C2, an equivalent series resistance R2, and an equivalent series inductance L2. The chip resistor 3 is defined by an equivalent resistance R3 and an equivalent series inductance L3, and the chip resistor 4 is defined by an equivalent resistance R4 and an equivalent series inductance L4. Furthermore, the circuit pattern on the circuit substrate B2 includes an equivalent inductance L, resistance R, and so on. Moreover, reference character S indicates a power supply voltage generating circuit which generates a power supply voltage, which is then supplied to the smoothing circuit on the circuit substrate B2. Reference character T indicates a load to which the voltage from the power supply voltage generating circuit S is applied via the smoothing circuit. In FIG. 3, the power supply voltage generating circuit S is provided separately from the circuit substrate B2, however, the power supply voltage generating circuit S may also be mounted on the circuit substrate B2.
FIG. 4 is a graph showing the relationship between the frequency and the impedance in the smoothing circuit shown in FIG. 3. The impedance of the smoothing circuit shown in FIG. 3 varies with respect to frequency as shown by line b. From this graph, it is clear that the impedance variation indicated by line b is small compared with the impedance variation indicated by line a (the same as in FIG. 2).
As can be understood from the structure of the circuit substrate according to the related art described above, even when a large-capacitance capacitor is used, it is possible to obtain a circuit in which the variation in impedance with respect to frequency is small, but, in this case, it is necessary to connect a resistance in series with the capacitor. However, when a chip resistor is mounted on the circuit substrate, since a series inductance component in the capacitor, due to the wiring lines, increases, faults such as deterioration of the decoupling ability occur. In addition, the required resistance is as small as several tens of milliohms to 100 mxcexa9, which is difficult to obtain as discrete resistance components.
Moreover, in order to form a resistance on the circuit substrate, a resistive film is disposed as a single circuit on the circuit substrate. Conductive particles such as aluminum oxide, tin oxide, tantalum nitride, etc. are used in this resistor film, these particles are mixed in either a glass or a binder to form a paste, the paste is applied to the substrate by printing or other process at predetermined positions and in predetermined shapes, and the substrate is baked at a high temperature, namely 600xc2x0 C. or above.
However, in this method, the resistance easily varies depending on the baking temperature. There is a problem in that the variation in resistance becomes large particularly in cases where it is necessary to control the temperature while baking.
In the related art disclosed in Japanese Patent No. 2578264, a metal oxide layer is formed on the surface of external electrodes, and by making this metal oxide layer function as a resistance, the equivalent series resistance (ESR) is increased, and the resistance is determined by the film thickness of the metal oxide layer. If a structure such as this is used, it is possible to obtain a small resistance, for example, from several tens of milliohms to 100 mxcexa9.
However, in the method for forming such a metal oxide layer, it is extremely difficult to control the oxidation of the terminal electrodes, and if the degree of oxidation increases slightly, the internal electrodes are also oxidized. As a result, there is a problem in that it is not possible for this structure to function as a capacitor.
In order to overcome the problems described above, preferred embodiments of the present invention provide a circuit substrate in which it is possible to obtain a circuit in which the variation in impedance with respect to frequency is very small, even when using a large-capacitance capacitor.
In a circuit substrate according to a first preferred embodiment of the present invention, in which an electronic component is mounted at a predetermined position of a circuit pattern, resistive films are disposed on the surfaces of lands included in the circuit pattern and these resistive films are provided as resistances which connect external terminals of the electronic component in series with the lands of the circuit pattern.
In the circuit substrate according to the first preferred embodiment of the present invention, since the resistive films are disposed on the surfaces of the lands included in the circuit pattern and these resistive films are used as resistances connected in series to the electronic component, a resistance can be connected in series without increasing the inductance in the electronic component. For example, in the case where the electronic component is a capacitor, even when a large-capacitance capacitor is used, a circuit having a small impedance variation with respect to frequency can be obtained. Therefore, it is possible to provide a power supply circuit and so forth having stable operation and high response speed.
Preferably, the resistive films used in the circuit substrate are made of a conductive adhesive.
In this circuit substrate, since the circuit pattern and terminals of the electronic component are connected by the conductive adhesive, which has a resistance component, the connection state is equivalent to that in which a resistance is connected in series without increasing the inductance in the electronic component. For example, when the electronic component is a capacitor, even when a large-capacitance capacitor is used, a circuit having a small impedance variation with respect to frequency can be obtained. Therefore, it is possible to provide a power supply circuit and so forth having stable operation and a high response speed.
Preferably, the conductive adhesive used in the circuit substrate has a specific resistance of approximately 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x922 xcexa9cm.
In this circuit substrate, since a conductive adhesive having a specific resistance of approximately 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x922 xcexa9cm is used, it is possible to obtain a large value for the resistance connected in series with the electronic component. Accordingly, it is possible to greatly reduce the impedance variation with respect to frequency.
Preferably, the conductive adhesive used in the circuit substrate has a resistance component in the direction that is substantially perpendicular to the surface of the circuit pattern, and the resistance can be set according to the film thickness of the conductive adhesive.
In this circuit substrate, it is possible to vary the resistance by regulating the film thickness of the conductive adhesive, which has a resistance component in the direction that is substantially perpendicular to the surface of the circuit pattern. Therefore, since it is possible to freely set the resistance of the conductive adhesive according to the type of electronic component, it can be appropriately set according to the circuit type, thus increasing the versatility.
Preferably, the conductive adhesive used in the circuit substrate preferably includes a combination of at least two types of conductive adhesives having different resistances. Electrical connection and mechanical connection are provided by one conductive adhesive having a smaller resistance, and the resistance component in the direction that is substantially perpendicular to the surface of the circuit pattern is provided by the other conductive adhesive having a larger resistance. The resistance can be set according to the film thickness of the conductive adhesives.
In this circuit substrate, by providing the electrical connection and mechanical connection using the low-resistance conductive adhesive, the electronic component can be electrically and mechanically connected. Moreover, since the resistance component in the direction that is substantially perpendicular to the surface of the circuit pattern is provided by the high-resistance conductive adhesive and the resistance can be set according to the film thickness of the conductive adhesive, the resistance of the conductive adhesive can be freely set according to the type of electronic component, and accordingly, it can be appropriately set according to the circuit type, thus improving the versatility.
Preferably, in the circuit substrate, the ratio of the resistances of the low-resistance conductive adhesive and the high-resistance conductive adhesive is approximately ⅔ or less.
In this circuit substrate, since the ratio of the resistances of the low-resistance conductive adhesive and the high-resistance conductive adhesive is approximately ⅔ or less, even if, for example, the variation in the CR components is about xc2x120% and the low-resistance conductive adhesive varies by about xc2x120%, in other words, even if there is about 40% variation, as long as the remaining high-resistance component does not vary by about 60%, the variation in resistance is within approximately xc2x120%. Therefore, the ratio of resistances is preferably about 40%:60%, that is, approximately ⅔, which ensures that the circuit operation is stable.
Preferably, the conductive adhesive in the circuit substrate preferably uses a filler including one material or a mixture of two or more materials selected from silver, copper, nickel, and carbon.
In this circuit substrate, by using a conductive adhesive in which the filler is one material or a mixture of two or more materials selected from silver, copper, nickel, and carbon, it is possible to obtain a resistance connected in series with the electronic component, and furthermore, by changing the type of filler, it is possible to vary the resistance.
Preferably, in the circuit substrate, the low-resistance conductive adhesive uses a filler including silver or copper, and the high-resistance conductive adhesive uses a filler including at least carbon.
In this circuit substrate, by using a conductive adhesive using a filler including silver or copper, a small resistance can be mounted, and by using a conductive adhesive using a filler including at least carbon, a large resistance can be mounted.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.